1. Field of the Invention
The invention relates to a method of producing a semiconductor device and, particularly, to a method of producing a semiconductor device using a chemical vapor deposition process in which a mixed gas containing a reactive source gas, a non-reactive solvent gas, and the like, is fed.
2. Description of the Related Art
As one of semiconductor devices, a ferroelectric memory (also called ferroelectric RAM (FRAM)), which is a non-volatile memory utilizing a capacitor using a ferroelectric film, is known. Such an FRAM is used as a memory in an IC card, or an alternative to a general purpose memory such as a DRAM or flash memory. In an FRAM, PZT (PbZrxTi1-xO3) or the like is used as a material for the ferroelectric film, and a lower electrode and an upper electrode are formed with the ferroelectric film therebetween.
The formation of a ferroelectric film for an FRAM is performed using a sputtering or sol-gel process. On the other hand, it is considered that a metal organic chemical vapor deposition (MOCVD) process, which is superior in the density and the step coverage of the deposited ferroelectric film, is promising for a purpose of obtaining a memory having an increased degree of integration. For example, when a PZT film is formed by an MOCVD process, it is known that, particularly, a lower electrode, on which the PZT film is to be deposited, is most appropriately an IrO2/Ir film, in that the lower electrode must have adhesion to the surface of a substrate in order to make a capacitor structure comprising a PZT film on an insulating film on the surface of a silicon substrate having a transistor fabricated therein, and diffusion of lead (Pb) and hydrogen to the lower electrode at an elevated temperature during the film formation must be prevented (Saito et al., Shin-Gaku Giho, pp31-35, March, 2000).
By way of example, the formation of a PZT film according to a prior MOCVD process using a flash vaporization is described by referring to FIG. 1. To feed, as source gases, metal elements (in this case, lead (Pb), zirconium (Zr), and Titanium (Ti)) needed to the film formation in the state of organometallic molecules, organic solvent solutions of organic metal materials for the respective metal elements are delivered from raw material containers 11a, 11b, 11c containing the respective solutions by introducing into the containers a pressure delivering gas, such as helium, from a pressure delivering gas tank 12, and are combined with a carrier gas, such as nitrogen gas, from a carrier gas tank 13 for the vaporization of the raw materials, to be introduced in a vaporizer 14 and to be vaporized. A mixed source gas comprising the vaporized organic metals and organic solvent is introduced to a mixer 16 through a line 15, where the mixed source gas is mixed with oxygen gas from a line 17 to be fed to a chamber 18 for the film formation. A heated substrate (not shown) is positioned in the chamber 18, where the organometallic molecules are decomposed at an elevated temperature and are reacted with oxygen to deposit a desired PZT film on the substrate. An exhaust gas comprising reaction by-product gases and non-reacted source gases is discharged from the chamber 18 by a vacuum pump 19, and is vented to atmosphere through an air pollutant eliminator 20. A line 21 from the vaporizer 14 to the vacuum pump 19 is used to allow the source gases to bypass the chamber 18 and to be vented through the air pollutant eliminator 20, during the non-formation of film.
In general, organometallic materials are supplied in the form of liquids having the organic metals dissolved in organic solvents, as described above, and the liquid material is vaporized to form a source gas and to be fed to a chamber for the film formation. The organic solvent is also vaporized and fed to the chamber together with the organic metals. As the organic solvent, an solvent allowing the concentration of the organic metal material in the liquid to be high, i.e., having a good solubility of the organic metal material, is used to efficiently supply the organic metal. For this reason, tetrahydrofuran (THF, (CH2)4O), butyl acetate (CH3COOC4H9) or the like are, in general, selected as the organic solvents.
As set out above, an IrO2/Ir film (which is made by forming an Ir film on a substrate and oxidizing the upper portion thereof) is provided on the surface of a substrate on which a film of a ferroelectric, such as PZT, is to be deposited. Various organic solvents, such as those used for dissolving an organic metal material for the deposition of a ferroelectric film, has a reducing power, and during the formation of PZT film on an Ir2O film, organic solvent molecules in a source gas fed onto a substrate act as a reducing agent for the IrO2 film on the substratre. By way of example, the change of orientation of crystal of an IrO2 film before and after feeding butyl acetate onto a substrate provided with the film, which was evaluated by X-ray diffraction (XRD), is shown in FIGS. 2A and 2B. The orientation peak of IrO2 (110) found before the feeding of butyl acetate (FIG. 2A) almost disappears after the feeding thereof, as seen in FIG. 2B, and it can be seen, in FIG. 2B, that the peak of Ir(111) is noticeable. This indicates that oxygen was expelled from the IrO2 film to leave an Ir film. The disappearance of the IrO2 film means that a barrier effect of the IrO2 film on the diffusion of lead and hydrogen during the deposition of a PZT film is lost, ultimately resulting in the cause of degradation of fatigue characteristics of the PZT film (stability of the amount Qsw of the switching charge after repeated polarization inversions).